The present invention relates DC to DC power converters for converting an input voltage and current to a different output voltage and current. In particular, the invention relates to switching power converters of the full bridge type which in the past have completed transitions from one switched state to another by means of dissipatively forced commutation.
Switched regulator DC to DC converter circuits are highly efficient. Examples of these include such converter topologies as the buck, (or step-down) the boost, (or step-up), and the buck-boost (or flyback), which may be used either for step-up or step-down voltage regulation.
All of these converters include at least one switching device, such as a bipolar transistor, and a regulating means, such as a semiconductor diode. In these switching regulator circuits, the storage inductor and the capacitor account for a major part of the regulator's cost and its electrical losses. In addition to these losses there are losses due to capacitance of the switching transistor. For example, if an FET is employed there are losses incurred as a result of charging and discharging the drain-source capacitance and also the gate-source capacitance. In addition, there are capacitance losses in the rectifier which may consist of a separate diode, or which may be implemented as an integral part of the FET.
Additional losses are created by the diode reverse recovery losses and FET losses due to finite transition times during switching. It is desirable to reduce all of these losses to a minimum. In particular, it is advantageous to eliminate the capacitive losses of the switching transistor and the rectifier, which occur during the transition from one switched state to another, due to dissipatively forced commutation in conventional power converters.
Since frequency modulation is used to control the output characteristics of a resonant converter, new techniques and methods must be developed for feedback control and stability analysis of such converters; furthermore, input and output filter design is more complex with resonant converters. Because internal waveforms have large sinusoidal components, the off-state voltage stress on semiconductor devices is increased and the conduction losses in both active and passive components are increased.
Various versions of resonant-transition, zero-voltage switching power converters are known which draw the energy required to charge the various switching transistor and rectifier capacitances from lossless reactive energy sources. For the buck, boost, buck-boost and certain combinations/extensions of these converters, this can be accomplished by placing a rectifier across the switching transistor, and then placing a second switching transistor across the diode rectifier. These transistors are then controlled by separate clock phases so that bipolar current flow is obtained over the complete range of the power converter from no load to full load. The current in the inductor in such a device always has a negative component, as well as a positive component, during each switching cycle from no load to full load. This results in the capacitance of the switching transistors and rectifiers of the circuit being reactively, rather than dissipatively, commutated.
While separate field effect transistors and diode rectifiers may be employed, it is also possible to employ field-effect transistors (FET's) or HEXFET's which have integral reverse rectifiers built into them. This HEXFET type of device is described in the article entitled "The HEXFET's Integral Reverse Rectifier--a `Hidden` Bonus for the Circuit Designer" by W. Fragale, B. Pelly and B Smith in Power Conversion International, March-April 1980, pages 17-36.
U.S. Pat. No. 4,186,437, issued Jan. 29, 1980, entitled "Push-Pull Switching Power Amplifier" to Slobodan M. Cuk described a converter which is generally known as a Cuk converter, which had a specific configuration that allowed for bidirectional power flow to obtain high efficiency, small size, and low weight. In addition, with this design there was reduced pulsation of both input and output currents and elimination of the switching ripple in the output. This converter, while utilizing bidirectional power flow, however, drew inductor currents which were only of a single polarity during a switching cycle.
Non-dissipative snubbing networks are known in which the load is inductive during turn-on of a power converter switch, and is capacitive during turn-off. the article entitled "A New Non-Dissipative Load-Line Shaping Technique Eliminates Switching Stress in Bridge Converters" by Ron Goldfarb in IEEE Proceedings on Powercon 8, D-4, pages 1-6 describes a buck-type, full bridge configuration which describes the use of such a snubbing circuit. The technique described in the converter of this article depends on using the magnetizing current of the output transformer to recover the charge on the snubbing capacitors. Current steering is accomplished by Goldfarb by independently controlling the conduction time of the power transistors. Four switching transistors, a diode bridge, a snubber circuit, a snubber reset sense circuit and a pulse steering logic circuit are employed.
Other patents and articles dealing with the reduction of losses in power converters are found in the following:
K. H. Liu and F. C. Lee, "Resonant Switches--A Unified approach to Improve Performance of Switching Converters," IEEE INTELEC Proceedings; pp. 344-351, 1984. PA0 K. H. Liu and F. C. Lee, "Zero-Voltage Switching Technique in DC-DC Converters," IEEE PESC Record; pp. 58-70, 1986. PA0 R. Goldfarb, "A New Non-dissipative Load-Line Shaping Technique Eliminates Switching Stress in Bridge Converters," Proceedings of Powercon 8, paper D-4, 1981. PA0 T. M. Undeland, "Snubbers for Pulse Width Modulated Bridge Converters with Power Transistors or GTOs," IEEE IPEC Record, pp. 313-323, 1983. PA0 R. P. Severns and G. Bloom, "Modern DC-to-DC Switchmode Power converter Circuits," Van Nostrand Reinhold, New York, pp. 19-23, 1985. PA0 H. C. Martin, "Miniature Power Supply Topology for Low Voltage Low Ripple Requirements," U.S. Pat. No. 4,618,919. PA0 R. D. Middlebrook and S. Cuk, "A General Unified Approach to Modeling Switching-Converter Power States," Advances in Switched Mode Power Conversion, Vol. I&II, Teslaco, pp. 73-89, 1983. PA0 C. P. Henze and N. Mohan, "Modeling and Implementation of a Digitally Controlled Power Converter Using Duty Ratio Quantization," IEEE/ESA PESC Record, ESA Proceedings, pp. 245-255, 1985. PA0 U.S. Pat. No. 4,672,303 in the name of Stephen F. Newton, issued June 9, 1987. PA0 U.S. Pat. No. 4,720,668 in the names of Fred C. Lee and Kwang-Hwa Liu, issued Jan. 19, 1988.